MEIOSIS

Chapter 8

Chromosome Pairs

Human somatic (body) cells consist of 23 homologous chromosome pairs Identical length, centromere

position, and loci Alleles are different versions

First 22 are autosomes Pair 23 are sex chromosomes

Few homologous parts XX = female and XY = male

Gametes (eggs or sperm) consist of 23 unpaired chromosomes

Karyotypes

Size ordered chart of all chromosomes in a cell Size Banding patterns Centromere location

Visualizes chromosomal makeup Matched up homologs

Cell Types

n = number of chromosomes in a single set Diploid cell has 2 homologous sets of

chromosomes One from each parent

If synthesis has occurred, each appears as sister chromatids Total number of chromosomes is the diploid number

(2n) Humans have 23 sets so diplod nubmer is (2 * 23 = 46)

Haploid cell has 1 set of chromosomes Called gametes Total number of chromosomes is the haploid number

(n) 22 autosomes and 1 sex chromosome (X or Y)

Egg always X and sperm is X or Y

Fertilization

Sperm (male) + egg (female) = zygote n + n = 2nHaploid + haploid = diploid

Meiosis makes gametes Only in reproductive areas Reduce chromosome number by half

Haploid cells keep number of chromosomes from doubling with each generation

Mitotic phase ensures that chromosome number is maintained

Meiosis (Overview)

Creates haploid gametes (n) in diploid (2n) organisms

2 phases Meiosis I (homologs separate) Meiosis II (sister chromatids

separate) Produces 4 daughter cells

with ½ number of chromosomes

Only time homologs associated with each other

Meiosis I

Prophase I 90% of meiotic time Synapsis is when the chromosomes pair

up 2 homologous duplicated chromatids = tetrad Crossing over exchanges genetic info

Similar to other processes in mitotic prophase

Metaphase I Tetrad arranged in middle of cell Spindle microtubules attach to the

centromere of 1 chromosome (2 sister chromatids)

Meiosis I (cont.)

Anaphase I Chromosomes move to

respective poles, splitting the tetrad

Telophase I Chromosomes reach poles

Haploid sets at each Cytokinesis

Occurs with telophase 1 Creates 2 haploid daughter cells

Interphase may or may not occur No duplication occurs regardless

Meiosis II

Like mitosis except it starts with haploid cells

Prophase II Spindles form and move chromosomes to

middle Metaphase II

Chromosomes align Anaphase II

Sister chromatids split and move to poles Telophase II

Nuclei form at poles Cytokinesis occurs

4 daughter cells (haploid)

Comparing Mitosis and Meiosis Mitosis

For growth, repair, and asexual reproduction Daughter cells are genetically identical 1 nuclear division = 2 daughter cells same # of

chromosomes Meiosis

Produces gametes for sexual reproduction Haploid daughter cells with a half of each homologous

pair 2 nuclear division = 4 haploid cells Meiosis I has all unique stages

Both Chromosomes duplicate only once Meiosis II and mitosis events

Sources of Genetic Variability Independent orientation at metaphase I (meiosis I)

Random fertilization

Crossing over in prophase I

Mutation is the ultimate source

Independent Orientation and Random Fertilization

Genetic diversity from reshuffling of genes during sexual reproduction

Occurs in metaphase I 1 chromosome from

mom and 1 from dad Tetrads randomly line

up on either side About equal probability

for all cases Total arrangements =

2n

Humans: 223 = 8 million 2 different gametes =

8 million2 = 64 trillion

Crossing Over

Exchange corresponding segments between homologs Occur at chiasma

Results in genetic recombination Varies from original

chromosome Happens average of 1 – 3

times in human meiosis

Errors in Meiosis

Abnormal chromosome numbers or size Nondisjunction, pairs of

chromosomes don’t separate Meiosis I = 4 abnormal cells Meiosis II = 2 abnormal and 2

normal Unknown what causes

Body generally miscarries when this occurs

Often leads to disorders or syndromes

Down Syndrome

Trisomy 21 Most common

chromosome abnormality and most serious birth defect in US

Incidence rate increases with age of mother

Generally don’t reproduce Sterile or chances of

passing to offspring

Sex Chromosome Disorders

Sex Chromosome Abnormalities

Genotype Gender Syndrome Physical TraitsXXY, XXYY, XXXY Male Klinefelter

syndromeSterile, small testicles, breast enlargement

XYY Male Jacob’s syndrome Normal male traits, previously thought ‘super-males’, karyotype ID only

XO Female Turner syndrome sex organs don't mature, sterile, short stature

XXX female Trisomy X Karyotype only identifier

Structural Abnormalities Duplication

Huntington’s disease Deletion

Cri du chat: developmental & neurological problems

Inversion Less likely to be harmful to

affected Increase miscarriage

Translocation Not between homologs Can cause cancers